DP Chemistry: 1.2 The mole concept

1.2 The mole concept

Written specifically for students to provide help and support for the IB Diploma chemistry programme this page provides full coverage of the syllabus content of Topic 1.2 The mole concept.  It encourages you to think critically and provides many questions with full worked answers so that you can monitor and improve your knowledge and understanding.

Learning outcomes

After studying this topic you should be able to:

Understand:

  • that a mole is a fixed number of particles and refers to the amount, n of a substance.
  • masses of atoms are compared on a relative scale to 1/12 of 12C and are expressed as relative atomic mass, Ar, and relative formula or molecular mass, Mr.
  • molar mass, M, has the units g mol-1.
  • the empirical formula of a compound gives the simplest whole ratio of the actual number of atoms present in one molecule of the compound whereas the molecular formula gives the actual number of atoms present in one molecule of the compound.

Apply your knowledge to:

  • calculate the molar masses of atoms, ions, molecules and formula units.
  • solve problems involving the relationships between the number of particles, the amount of substance in mol and the mass in grams.
  • determine the empirical formula of a compound from its percentage composition by mass and vice-versa.
  • determine the molecular formula of a compound from its empirical formula and its molar mass.
  • obtain and use experimental data for determining empirical formulas from reactions involving mass changes.

Relationships & vocabulary

Nature of Science

The concept of the mole developed from the concept of ‘equivalent mass’. The equivalent mass of an element was the mass required to react with one gram of hydrogen. Dalton’s idea of atoms could not account for the fact that 8 g of oxygen combines with 1 g of hydrogen to form 9 g of water whereas from Avogadro's and Gay-Lussac’s experiments, two volumes of hydrogen gas combine with one volume of oxygen to produce two volumes of water. Avogadro’s hypothesis is an example of a paradigm shift to explain this in terms of molecules.

International-mindedness

The SI system (Système International d’Unités) refers to the metric system of measurement. It is based on seven base units: metre (m) for length, kilogram (kg) for mass, seconds (s) for time, ampere (A) for current, kelvin (K) for absolute temperature, mole (mol) for amount of substance, and candela (cd) for luminous intensity. The unit g mol-1 is a derived SI unit.

The International Bureau of Weights and Measures (Bureau International des Poids et Mesures -BIPM) aims to ensure worldwide uniformity in the application of SI units.

For examples and more links to International mindedness, Theory of knowledge, utilization etc. see separate page which covers all of Topic 1: Stoichiometric relationships.

Vocabulary

mole (mol)moleculeAvogadro's constant (L or NA)

Learning slides

You can use this slide gallery for learning or for reviewing concepts and information. It covers all the key points in the syllabus for this sub-topic.

  

Something to think about

Although Avogadro’s number is probably the best known number in chemistry, Avogadro himself knew nothing about its value or the concept of the mole. Avogadro, or to give him his full title, Lorenzo Romano Amedeo Carlo Avogadro, conte di Quaregna e di Cerreto was a lawyer with an interest in philosophy. He was born in Turin in 1776 and around the turn of the nineteenth century began private studies in science. This was the same time as John Dalton was proposing his atomic theory. Avogadro worked on gases and showed that the ratios of volumes of reacting gases were simple numbers. In 1811 he published a paper in Journal de physique in which he stated that the particles of simple substances such as hydrogen, oxygen and nitrogen were composed not just of atoms (as Dalton claimed) but of atoms combined together in molecules. He reasoned correctly that one molecule of oxygen combines with two molecules of hydrogen to form two molecules of water. He went on to propose what is now known as Avogadro’s Hypothesis: equal volumes of different gases (under the same conditions) contain the same number of molecules. Because he was relatively unknown Avogadro’s work was ignored for almost fifty years until in 1860 (four years after Avogadro’s death) Stanislao Cannizarro explained its importance, particularly how it could be used to determine atomic and molecular masses.

Test your understanding of this topic

(Note that your teacher may have restricted your access to some or all of these questions and worked answers if they are going to use them as a class test or set them as an assignment.)

For ten 'quiz' multiple choice questions with the answers explained see MC test: The mole concept.

For short-answer questions see Empirical & molecular formula questions and a separate set of short-answer questions The mole & Avogadro's constant questions.

More resources

1. A PowerPoint which gets you to 'guesstimate' the number of grains of sand on a beach and relates this to one mole.

Guesstimating sand

2. A separate page on this site Comprehending Avogadro's constant (which you have direct access to) which may help you to grasp the true size of Avogadro's number.

3. A video by Mad Penguin Films that attempts to illustrate how big a mole is - put to music.


  How big is a mole

All materials on this website are for the exclusive use of teachers and students at subscribing schools for the period of their subscription. Any unauthorised copying or posting of materials on other websites is an infringement of our copyright and could result in your account being blocked and legal action being taken against you.